Abstract [en]

Metal transport in small streams in boreal catchments is a function of weathering rate, water balance and redistribution mechanisms. Because of these highly dynamic processes long term water quality changes are difficult to determine but needed in order to assess the impact of several local and largescale changes on local water quality.

The field site is situated at Dyltabruk, some 20 km North of Örebro in South Central Sweden. The 4 km2 catchment has deciduous and coniferous species on a granitic moraine with some 20% ofmires and fens adjacent to the oldest sulphur mine in Sweden. Grab samples were collected weekly since 2006 but more frequent during periods with large changes in water balance. The samples were analysed for general hydrochemical parameters (temperature, electrical conductivity, pH, dissolved organic carbon (DOC), inorganic carbon (IC), fulvic and humic acids and dissolved oxygen), dissolved principal anions, principal and trace metals. Standardized analytical procedures were applied. Temperature, precipitation and other climatic parameters were recorded some 2 km from thesite every 15 minutes.

The results showed a general concentration pattern where the water balance had the largest single influence. The concentrations had a seasonality inversely related to the ground water level. Inter annual variations of one to two orders of magnitude were observed for group 1 and 2 elements. For transition metals with high affinity to solid matter as well as DOC the variation reached three to four orders of magnitude. Only aluminium and iron had concentrations that occasionally exceeded solubility limits which resulted in a similar inter annual variation.

During the study period the average annual temperature and precipitation were no different (p 0.05) from the previous ten years but rainfall intensities increased. In a long term perspective the concentrations for all metals except calcium had positive trends. The tendencies remained when normalizing against chloride. The same was found for DOC, nitrate and sulphate. Hence, there is an accelerating loss of most elements that is not limited by weathering. It is uncertain, however, if the positive trends for DOC depend on increased production or a balancing release from the supply in mires and fens. In addition, there is also an indication of increasing inter annual concentration changescwith time. Although not exclusively proven such phenome would occur as a result of increased rainfall intensity. It is therefore likely that the accelerating loss of elements is a result of increased weathering rather than increasing water discharge.